The problem with very high reps and light weight is that they only target your Slow-Twitch muscle fibres. Majority of people already get enough stimulation in their slow twitch muscles just by doing everyday activities, even if they don't lift weights... Most people neglect their Fast-Twitch type IIA and IIB muscles, because they never use them, but it's extremely important to regularly activate your motor units which control your fast twitch muscles and the only way to do that is through heavier weights and lower reps and also through sprinting and plyometric training.. Fast twitch muscles are your strongest muscles and they are also the very first to suffer from degradation due to ageing...Slow twitch muscles are with you forever and you never loose them ,fast twitch muscles are lost unless regularly used, if you don't use it you loose it..Do high reps for few weeks then switch to lower reps, and alternate between low and high every few weeks.